MicroRNAs (miRNAs) are a recently discovered class of small 20-22 nucleotide non-coding RNA molecules that have been shown to regulate target gene expression in various organisms. By targeting the mRNA of protein-coding genes for either cleavage or repression of translation, miRNAs are thought to play critical roles in development, control of growth, proliferation, and cell lineage determination. However, the ability of this new class of regulatory RNAs to influence the processes of proliferation and differentiation in malignancy remains to be uncovered. Using an oligonucleotide array designed to detect the majority of mammalian miRNAs identifies thus far, we measured the expression levels of miRNAs in glioblastomas. This preliminary study identified a cluster of miRNAs that is up-regulated in glioblastomas. Among this cluster one miRNA is markedly elevated. Sequence-specific inhibition of this miRNA with modified antisense oligonucleotides induces apoptosis of glioblastoma cells in culture, suggesting a role for this miRNA in gliomagenesis. Similar regulatory miRNA molecules unique to tumor cells may exist and can serve as prognostic markers and, probably, excellent therapeutic targets for the treatment of high-grade brain tumors. The long-term goal of this proposal is to treat glioblastomas by targeting miRNAs. The immediate goal over the next two years is the development of technologies required for identification and validation of miRNA targets. We will create an oligonucleotide array for the complete profiling of miRNA expression in glioblastomas and characterize molecules expressed exclusively in these tumors. We will then develop ways to suppress these miRNAs in glioblastoma cell cultures and assess the effects on the migratory, apoptotic, and proliferative capacity of the tumor cells. Furthermore, we will test the potential of the miRNA inhibitors in animal models in vivo. Although beyond the scope of this proposal, in the ensuing years we will study downstream mRNA and protein targets that mediate miRNA functions. The results of the experiments proposed here will improve our understanding of biology of brain rumors. Moreover, miRNA targeting technology developed in this research could open the door to novel treatments of glioblastoma.